Haruna Sakurada - Usagi's long-suffering English teacher. Affectionately called "Haruda" by her students, she is a decent teacher - she looks out for her students' welfare, warning them to be cautious in dangerous situations. She puts up with a lot from Usagi - besides falling asleep in class, getting to school late, and failing major exams, Usagi also has a tendency to bring cats into class. (Tidbit: Haruna-sensei was also the teacher of Asuka Chieri, heroine of Naoko-san's Cherry Project series; in Japanese, "Sakura da, haru da" means "Cherry blossoms, it's spring.")
Osaka Naru - Naru-chan is Usagi's best friend at school. She's much more ladylike than Usagi, which means that she's not at all clumsy, not much of a crybaby, and gets good grades at school. She's demure, responsible, and quite popular; apparently she's easy to get to know. She's usually on top of the latest fads, and well-informed about the local gossip. (Unlike the anime, she has no love interests to speak of.) Her family owns the local "Osa-P" jewelry store. It was concern for Naru-chan that spurred Usagi to action when she first became Sailormoon.
Umino Gurio - a nerd. Umino is blond, short, excited, and a bit of an alarmist. He's full of the latest gossip, and tends to draw hasty conclusions. He gets good grades, but brags that school is just a game to him. He looks at the world through thick glasses, and is about as impressionable as a blob of hot wax.
Kuri and Yumiko - Two ordinary schoolgirls at Uasgi's Junior High. They basically exist to have things happen to them. Yumiko bought energy-draining jewelry from Jaedite's youma at the Osa-P store; Kuri was lured by the false promises of the Crystal Seminar cram school. Both girls are otherwise normal; they get decent grades, love jewelry, and enjoy ice cream. Usagi eats lunch with them, and they share juicy bits of gossip.
Asanuma Ittou is a boy slightly younger than Mamoru; he goes to Mamoru's elite Moto Azabu school. Asanuma's a perceptive, observant lad who idolizes Mamoru. (Makoto's also more than slightly interested in this boy.) Asanuma calls Mamoru and Makoto "senpai" because they're older than he is, and sees Mamoru as a sort of big brother. Asanuma-kun is a member of the science fiction club at Moto Azabu, and posesses an active imagination. He saw Mamoru heal himself with a touch, which led him to hypothesize that Mamoru (and Makoto, and all the other senshi) were superhumans or aliens or some such. After finding out what they truly were, he longed to be able to protect Makoto from any danger; Makoto thought that he was a sweet boy and kissed him on the forehead for it. Unfortunately, after the Black Moon is defeated, Asanuma-kun is never heard from again; I for one would have liked to see him get together with Mako-chan. Asanuma is slim and blond, and his eyes are intense.
Oops! I've been corrected. Asanuma does make one more appearance, in Stars 3. Usagi calls him, asking whether Mamoru has contacted him. (Mamoru hasn't, being dead at this point.) Asanuma admits that he hasn't heard from Mamoru either, but does his best to reassure Usagi. The fact that Usagi feels comfortable contacting Asanuma shows that the senshi still keep in touch with him, which is pretty cool; also, the bond between the senpai and the kouhai is evident in that Mamoru is still expected to keep in touch with Asanuma.
Chief Sarashina Kotono is the head of the supernatural phenomena study group at Rei's T-A Private Girls' School. (In Japanese, "Chief Kotono" is "Kotono-bucho.") Her pet project is studying UFO's. She really wanted Rei to be a part of her group, but Rei was a little reluctant. (Rei thought UFOs were silly.) If anything, Kotono's imagination is even more active than Asanuma's. She tends to get really worked up about strange and unusual happenings. Her hair is pale blond, tied back from her face by two bowties.
An excellent site about Kotono-bucho and Asanuma-kun: